Gate Controlled Atomic Switch

ABSTRACT

The invention relates to a method for producing a switch element. The invention is characterized in that the switch element comprises three electrodes that are located in an electrolyte, two of which (source electrode and drain electrode) are interconnected by a bridge consisting of one or more atoms that can be reversibly opened and closed. The opening and closing of said contact between the source and drain electrodes can be controlled by the potential that is applied to the third electrode (gate electrode). The switch element is produced by the repeated application of potential cycles between the gate electrode and the source or drain electrode. The potential is increased and reduced during the potential cycles until the conductance between the source and drain electrode can be switched back and forth between two conductances, as a result of said change in potential in the gate electrode, as a reproducible function of the voltage of the gate electrode.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/158023, filed on Jun. 10, 2011, which is a continuation of U.S. Pat.No. 7,960,217, which is International Application PCT/DE2005/001541,which claims priority to German Application No. 10 2004 043 811.0, allof which applications are incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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MICROFICHE/COPYRIGHT REFERENCE

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STATE OF THE ART AND PRESENTATION OF THE PROBLEM

The development in microelectronics is characterized by an increasingminiaturization. In addition to a miniaturization of dimensions ofindividual components, in particular of transistors and the transitionto increasing frequencies [1, 2] also the reduction of energyconsumption per logic operation comes increasingly to the fore. Insemiconductor structures of processors and memory chips which arenowadays produced the dimensions of the individual components of amicrochip are already less than 100 nanometers with the purpose offurther miniaturization. While the semiconductor technology is stillbased on silicon based systems to a large extent, fornanoscale-electronics alternative systems are also discussed morefrequently, in particular the design of logic elements such as switchesand transistors based on individual molecules (so-called molecularelectronics) [3, 4, 5].

There has been hardly any discussion on the possibility of the design ofelectronic circuits based on components, the active structural unit ofwhich are not individual molecular structures (partly special and partlycomplex), but individual atoms, for example metal atoms (“atomicelectronics”). While in the case of molecular electronics there are alarge number of proposed concepts but also of experimentalimplementations already available, there is not yet a concept for atomicelectronics. While passive components such as capacitors and resistorson an atomic scale have been implemented and investigated experimentallyas prototypes for a long time, atomic electronics has failed so far onthe implementation of an atomic transistor, i.e. a component on anatomic scale, in which a source-drain resistance specifically will becontrolled by an independent third electrode, the gate electrode, andfor instance can be specifically controlled/switched by means of thevariation of a potential applied on the gate electrode between anelectrically conducting on-state and a electrically less conducting orideally non-conducting off-state.

On the other side, considerable preliminary research has already beendone on the fabrication of contacts between individual atoms [6, 7, 8,9, 10, 11, 12], which is achieved in a mechanical way, whereby thinmetallic bridges will be stretched out to such an extent that thecontact area will be made up of a single or a few atoms in diameter. Inthis process, in particular mechanically controllable break contacts(Mechanically Controllable Break Junctions, MCB) and the contact betweenthe metallic tip of a scanning tunnel microscope and a metallic samplehave been used, but also contacts in relays and others have beeninvestigated. It could also been demonstrated that metallic pointcontacts on an atomic scale can be established by means of galvanicdeposition of metals from an electrolyte into a small gap between twoelectrically conducting contacts [10, 11, 12]. While such contactsfrequently but not always turn out to be quantum point contacts havingconductance values of integer multiples of the conductance quantum,their conductance values, which they adapt, can hardly be predeterminedor adjusted beforehand on a predefined value. In fact, the conductancevalue of the metallic bridge is decreasing when its diameter isdecreasing successively, mostly in several stages, until the bridge isbreaking. The essential problem of the implementation of atomic ormolecular electronics, i.e. the implementation of active componentswhich made it possible by means of an independent third controlelectrode to control and adjust specifically the conductance valuebetween source and drain electrode, is not yet solved therewith.

In the past there had been two approaches for solving this problem. Asfor one approach, an atomic contact was repeatedly opened and closedwhile two macroscopic electrodes were moved towards one another andafterwards moved away from another [13]. In this process without anydoubt the contact on an atomic scale was established, however, theopening and closing of the contact required the movement of amacroscopic electrode.

As for the second approach, the group of Don Eigler [14] succeeded inswitching the position of a single atom in a tunneling microscopebetween two positions (on the tip of the tunnel and on the surface ofthe sample). In this case there is a component the only movable or movedpart of which is a single atom. This atom flip-flop has not only thedisadvantage that it operates in the shown configuration only at lowtemperatures (typically at 4 K up to 30 K) and in ultra high vacuum,that is, not under conditions where in technical applications electronicrelays operate. Moreover, there is also no independent third electrodeas a control electrode or gate available, instead the switching of theatom position of the movable atom is achieved by applying a potential toboth electrodes the conductance value of which has to be switched.However, first and foremost, this arrangement does not allow to open andclose an electrical circuit, but the resistance of the contact typicallyvaries between 0% and 40% due to the switching of the position of theatom, whereby this percentage of variation cannot be predicted exactly.

A different relay element has been described by Fuchs and Schimmel [15,16] with a switching process on an atomic scale. Unlike the abovedescribed device, in this element the switching process can also becarried out under ambient conditions, i,e. at room temperature and inair, i.e. without the necessity of vacuum or exclusion of oxygen.However, there is mainly a positional switching. Switching of anelectrical tunnel current between a higher and a lower value can alsoonly be observed if a tunnelling microscope is used. The tunnel currentcannot be switched on and off by means of the atomic element.

EXPLANATION OF THE PROCEDURE AND OF THE COMPONENT ACCORDING TO THEINVENTION

By the procedure according to the invention this problem is solved as aatomic switching element has been designed, the only moveable elementsof which are the contacting atoms and the electrical contact of whichbetween two electrodes (which are called source and drain) can bespecifically opened and closed by means of an potential which is appliedto an independent third electrode (control potential). This componentoperates at room temperature and without exclusion of oxygen. The ratioof the source drain-conductance in the on and off-state can be more than1000, and according to the embodiment more than 10,000.

The fundamental idea of the process according to the invention is thetraining of an electrochemically produced atomic point contact byrepeated cycling in the following manner:

At first in a small gap between two electrodes metal will be depositedgalvanically from an electrolyte until the contact between both of theelectrodes is closed and a pre-adjusted upper conductance value isexceeded. Afterwards immediately or after a defined delay a dissolutionpotential V2 will be applied to both of the electrodes with respect tothe reference electrode (it may be but need not be carried out, forinstance, not be varying the potential of both of the gold electrodes,but through varying the potential of the quasi reference electrode withrespect to a reference potential “ground”), until the conductance valuefalls short of a lower conductance value Y, and then a depositionpotential V1 t will be applied again until in the contact the upperconductance value is reached and the cycle of applying the dissolutionpotential V2 starts again.

This procedure will be repeated until by this training of the contact asa response to applying a dissolution potential to the working electrodewith respect to the reference electrode (the potential of the workingelectrode has a positive bias relative to the reference electrode) theconductance value of the contact with or without a delay jumps to thevalue “zero” and by applying a deposition potential (the potential ofthe working electrode has a negative bias relative to the referenceelectrode) the conductance value of the source drain-contact with orwithout a delay jumps to the intended value G. The described procedureis working in an especially advantageous manner, when the intendedon-state-conductance value G is a multiple of the conductance quantum.

By means of a hold-potential, i.e. a value of the potential which isbetween deposition and dissolution potential, a given conductance value(on-state or off-state) can subsequently be hold constant as long as bychanging the potential it will specifically be switched via thedeposition potential from the off-state to the on-state or via thedissolution potential from the on-state to the off-state.

Thus the function of a transistor or of a relay on an atomic scale canbe implemented. This component is an atomic switch or an atomic relaywhich can be used as a functional unit for atomic logic switches andlogic chips as well as for atomic electronics.

After setting a defined on-state and then setting the cycling andswitching and applying a hold-potential only (the value of which maygenerally also be different for holding the on-state and holding theoff-state, see the embodiment described below as an example) thisprocedure cannot only be used for fabricating and operating of atomicswitches and atomic transistors but also for fabricating of a resistorwith a pre-selectable value, i.e. with a given value defined beforefabrication, which may be preferably an integer multiple of theconductance quantum.

An embodiment of the above described application will be describedbelow. Further embodiments will be described in appendix 1, appendix 2and appendix 3.

The present invention will be explained further in detail with respectto the following embodiments but is not restricted to them.

FIGURE LEGENDS

FIG. 1 (a) is an illustration of the fundamental principal of a metalquantum point contact based switching on an atomic scale. The contactingatoms are moved back and forth by an externally applied gate potentialresulting in a gate potential controlled closing and separating of thecontact on an atomic scale.

FIG. 1 (b) schematically shows the experimental set-up. In thisembodiment by applying a gate potential controlled electrochemicaldeposition potential silver is electrochemically deposited into thenano-scale gap between the gold electrodes (source and drain), while atthe same time the conductance is recorded between the gold electrodes bya measuring voltage typically of 12.9 mV. By repeatedcomputer-controlled electrochemical cycling a bi-stable switch on anatomic scale is generated.

FIG. 2: Switching of the conductance value by means of a controlpotential U_(control) by varying the control potential (a) theconductance value of the atomic silver contact (b) is switched between anon-conducting off-state and an on-state having a quantized conductancevalue of 1 G₀. The curves are the non-filtered measuring data and show asharp transition between these two states. This experiment illustratesan atomic switch which is externally controlled by a control potential.

FIG. 3: Time-dependence of the switching process: the figure shows thedeclining edge of the conductance value as a function of time during thedissolution process of the atomic silver contact. This section is a partof a longer sequence of periodic switch processes between theconductance values of zero and 2 G₀. The switching process starts with apre-phase of about 50 μs which is followed by the intrinsic switchingprocess within a time period of less than 14 μs.

FIG. 4 shows the switching of the conductance value between zero and apre-selected higher conductance value of 3 G₀. The conductance value ofthe atomic switch (b) is directly controlled by means of the controlpotential U_(control) (a) which is applied between the electrochemicalcontrol electrode and the gold working electrodes. If the controlpotential is put to a “halt-level” (see arrows), the atomic switchsteadily remains on its conductance level.

DETAILED DESCRIPTION OF THE EMBODIMENT 1 Experimental Set-Up/Preparation1.1 Measuring Set-Up

FIG. 1 schematically shows the measuring set-up for electrochemicaldeposition of atomic metallic contacts. There is an electrochemical cellfilled with an electrolyte of metallic ions and equipped withpotentiostatically controlled electrodes. Two gold electrodes whichserve as working electrodes are fixed to a glass substrate and areelectrically insulated by a distance between each other of about 100 nm.Both of the gold electrodes are insulated against the electrolyte bymeans of a polymer coating except for a microscopic area of the contactregion.

By applying an electrochemical potential difference between both of theworking electrodes and a quasi-reference electrode metal islands (in thepresent embodiment silver islands) will be deposited on the free area ofthe gold electrodes. At the same time the conductance between theworking electrodes will be recorded. This will be carried out as long astwo metal islands which have grown on two different gold electrodes comeinto contact to each other and will close the gap between both of thegold electrodes in an electrically conducting manner.

1.2 Electrochemical System for Deposition of Silver

For the electrochemical deposition of atomic contacts of silver anelectrolyte of an aqueous silver nitrate solution (0.1 mM AgNO₃+0.1 MHNO₃ dissolved in bi-distilled water) was used. Silver wires of 0.25 mmin diameter with 99.9985% purity serve as quasi-reference electrode andcounter electrode.

1.3 Electrochemical Deposition of Atomic Silver Contacts

In order to deposit silver a positive control potential between 2 mV and40 mV will be applied to the quasi-reference electrode; this willcorrespond to a deposition potential between −2 mV and −40 mV (each vs.Ag/Ag⁺) at one of the working electrodes [here it is called goldelectrode (1)]. The other working electrode which is called goldelectrode (2) is constantly on a potential which is lowered withU_(measur) as compared to the gold electrode (1).

In this embodiment a measurement potential U_(measur) of −12.9 mV wasapplied. This means that the deposition potential of the gold electrode(2) is 12.9 mV lower than the deposition potential of the gold electrode(1): as the deposition potential of the gold electrode (2) has anegative bias to the electrode (1), tendentially more silver will bedeposited on (2).

In order to generate atomic contacts now through applying a positivecontrol potential—that corresponds to applying a deposition potential tothe working electrodes—silver will be deposited on both of theelectrodes as long as two silver islands get in contact to each otherand these islands connect both of the gold electrodes in a conductingmanner. This will be checked by a continuous measurement of theconductance value between both of the gold electrodes during the processof deposition of silver. By means of a specially developed computerprogram the deposition at a given conductance value can be stopped orthe contact can be separated by application of a negative controlapplication—that corresponds to applying a dissolution potential to thegold electrodes.

In this manner electrochemically deposited atomic silver point contactshaving quantized conductance values can be generated. The measurementsare carried out at room temperature. The conductance value of the atomicsilver contact was ca. 1 G₀. After the deposition of the silver contactthe control potential was lowered to −29 mV. [For the sake ofclarification, this corresponds to an electrochemical dissolutionpotential of +29 mV vs. Ag/Ag⁺ of the gold electrode (1) or of (+29mV−12.9 mV=16.1 mV) vs. Ag/Ag⁺ of the gold electrode (2)]. As aconsequence of the separation of the contact the conductance value jumpsto the value of zero.

After increasing the control potential to +2 mV silver was depositedagain on the working electrodes as long as a new contact had beengenerated and thus the conductance value increased again to the value of1 G₀. Afterwards the deposition was stopped. The deviation of themeasured conductance value from the exact value of G₀=2e²/h was lessthan 1% in this case.

2 Atomic Switching 2.1 Specific Atomic Switching “Training” of ContactConfigurations by Cycling

In order to generate bi-stable contacts a procedure was used where anatomic contact was “trained” by several cycles of electrochemicaldeposition and dissolution, i.e. different contact configurations aregenerated until a bi-stable configuration appears. For this purpose acomputer program was developed by which the corresponding parameters canbe pre-selected and the cycling process can be carried outautomatically.

In the following an example will be described for generating a switchbetween the conductance value of zero and 1 G₀. At first an atomiccontact was deposited. As soon as the conductance value had reached anupper threshold (in the present case 0.94 G₀) which was close to thedesired conductance value of the on-state (1 G₀) the deposition wasstopped and the following computer-controlled cycle was started: byapplying a control potential for the dissolution process the contact wasseparated until the conductance value dropped to a lower limit(off-state, in the present case 0.05 G₀). Then a control potential fordeposition was applied again until the conductance value exceeded theupper threshold. Afterwards a new dissolution/deposition-cycle wasstarted, and so on.

During the first dissolution/deposition-cycles of a contact which isjust being generated anew often fluctuations of the conductance valuesoccur during the different cycles. Normally in the course of time atransition occurs spontaneously from an irregular fluctuation of theconductance value to a control potential controlled switching betweentwo levels (in the present case between the value of zero and the valueof 1 G₀).

Periodic Switching

FIG. 2 shows an embodiment for a sequence of five switching processes ofan atomic switch which was generated by the above described procedure.The atomic silver contact switches between an off-state having aconductance value of zero and an on-state having a conductance value of1 G₀ and is controlled by applying an external electrochemical controlpotential. This control potential is shown as a function of time in FIG.2 (a), while FIG. 2 (b) shows the simultaneously measured conductancevalue. Each change of the control potential is followed by a switchingof the conductance value of the atomic silver contact.

The individual switching processes occur in a very reproducible manner.In the case of the switch shown in FIG. 2 more than 1000 controlpotential controlled switching processes between the conductance valueof zero and G₀ could be observed. Furthermore, such switches withseveral different switches could be reproduced. If 1000 switchingprocesses are evaluated, an accuracy of reproduction of 0.8% (standarddeviation) is obtained of the conductance values, which are achieved inthe individual switching processes. The noise of the quantized on-stateis less than 0.4%. The deviation of the mean value of the measuredconductance from the theoretically predicted value of 1 G₀ is only 1.0%.The ratio of conductance values of the on- and off-state is limited insuch a manner that the conductance value of the off-state is not exactlyzero due to electrochemical leakage currents. Depending on theindividual configuration of the contact typical ratios between 1000 and3000 are achieved. The intrinsic switching process of the conductancevalue does not occur immediately after applying the control potential,but there is a certain delay between the change of the control potentialand its effect on the contact. This characteristic time depends on thecontact geometry and the ion concentration of the electrolyte and issome seconds for the present experimental set-up.

The intrinsic switching time of the transition, however, issubstantially shorter, as FIG. 3 shows. The declining edge of aswitching process of a reproducible sequence of transitions between theconductance value of zero and the conductance value of 2 G₀ is shownwith a time resolution in the μs-range. Initially the conductance valueruns almost constantly at 2 G₀. The real switching process starts in apre-phase of about 50 μs (t₀ in FIG. 3) during which the conductancevalue drops to about 1.7 G₀ and then the real switching process (t₁)occurs.

For the time of the real switching process (t₁ in FIG. 3) only an upperlimit of 14 μs can be reported due to the low time resolution of themeasuring electronics. In further experiments with improved electronicsswitching processes with a time period of ≦3 μs could be observed. Thismeasured time period, however, is still limited by a time resolutionwhich is still too small. The intrinsic switching velocity may be muchhigher, as—contrary to other procedures—the only movable parts of theswitch are individual atoms and therefore the physical limits of theswitching frequencies are in the Tera-Hertz range [17].

Specific Controlling

By means of the just described method of training of an atomic switchconfiguration not only switches can be produced having conductancevalues between zero an 1 G₀, but also switching processes can begenerated between conductance values of zero and other selectableinteger multiples of G₀. A section of such a generated contact, whichswitches between the values of zero and 2 G₀ was already shown in FIG.3.

A further embodiment is shown in FIG. 4: What is crucial is the choiceof the upper threshold value for the cycles of electrochemicaldeposition and dissolution of the contact. If for instance a switch isto be generated between the values of zero and 3 G₀, an upper thresholdvalue of almost 3 G₀ has to be chosen. As a consequence of the trainingprocess a contact is formed the conductance value of which will beswitchable between the values of zero and 3 G₀ by means of an externalcontrol potential (see FIG. 4). The shape of the signal, with which thecontrol potential is applied as a function of time (in this case theshape of a triangle) has no influence on the switching operation of theconductance value which proceeds between two values in a digital manner.

FIG. 4 shows a further possibility to interrupt the periodic switchprocess and to keep constant a defined conductance level. For thispurpose a halt-potential (in the present case −14 mV) is applied, whichis chosen in such a manner that at the contact a local electrochemicalequilibrium potential is established. This potential has the effect thatlocally no further electrochemical deposition of atoms or dissolution ofthe atomic contact occurs and thus the conductance value remainsconstant as a function of time. FIG. 4 shows this behaviour of theoff-state (arrow on the left) and of the on-state (arrow to the right).The atomic switch can therefore by means of controlling by the controlpotential operate in three different modes: Switching on the current,switching off the current, keeping the at last adopted state. Such aswitch is therefore the basis of logic switch elements on an atomicscale.

2.2 Summary of the Experiments for a Bi-Stable Switching

Along with the presentation of embodiments a procedure was described forproduction of a bi-stable atomic switch by means of cycles ofelectrochemical deposition and dissolution between two threshold valuesof conductance. These embodiments represent the first atomic switcheswhich are controlled by an external control electrode and in which theonly movable elements are individual atoms.

1. (canceled)
 2. A switching element comprising three electrodes: a source electrode, a drain electrode, and a gate electrode, said source electrode and said drain electrode being interconnected by means of a contact including a bridge made up of one or more atoms which can be reversibly opened and closed, the opening and closing of the contact between said source electrode and said drain electrode being controllable by an electrical potential applied to said gate electrode, said contact having an on-state and an off-state.
 3. A switching element according to claim 2 and further including and wherein said bridge is formed of a movable cluster of metal atoms of up to 100 nm.
 4. The switching element of claim 3 wherein said movable cluster of metal atoms includes two or more different metals.
 5. The switching element of claim 2 wherein said one or more moveable atoms includes two or more different metals.
 6. The switching element of claim 2 wherein said bridge is formed of two different metals.
 7. A switching element according to claim 2 and further comprising an electrolyte.
 8. A switching element according to claim 7 wherein said electrolyte is a liquid electrolyte.
 9. A switching element according to claim 7 wherein said electrolyte is a gel electrolyte.
 10. A switching element according to claim 2 and further comprising an ion conductor.
 11. A switching element according to claim 2 and further comprising an ionic system having movable ions.
 12. A switching element according to claim 6 wherein said ionic system is incorporated into a polymer.
 13. A switching element according to claim 6 wherein said ionic system is incorporated into a porous system.
 14. A switching element according to claim 2 and further comprising a polyanion having mobile cations.
 15. A switching element according to claim 2 and further comprising a polycation having mobile anions.
 16. The switching element of claim 2 wherein said bridge has a source-drain conductance value.
 17. The switching element of claim 16 wherein said source-drain conductance value is switchable between a zero value and a non-zero value.
 18. The switching element of claim 16 wherein said source-drain conductance value is switchable between two non-zero conductance values.
 19. The switching element of claim 16 wherein said source-drain conductance value is switchable between levels which are interger multiples of the conductance quantum 2e²/h, where e is the electron charge and h is Planck's quantum.
 20. A switching element according to claim 2 and wherein the switching element operates at room temperatures.
 21. A switching element according to claim 2 and wherein the switching element operates at temperatures between −30° C. to 50° C.
 22. A switching element according to claim 2 wherein said switching element is used as a transistor.
 23. A switching element according to claim 2 wherein said switching element is used as an atomic relay.
 24. A switching element according to claim 2 wherein said switching element is used in a logic switch for carrying out a logic operation.
 25. A switching element according to claim 2 wherein said switching element is used as a data storage device.
 26. A switching element according to claim 2 wherein said switching element is used as relay in connection with ultrahigh frequencies at least in the megahertz range.
 27. A switching element according to claim 2 wherein said switching element is used as a conductance value standard.
 28. A switching element according to claim 2 wherein said switching element is used as a resistor standard. 